Seal for an electrochemical cell

ABSTRACT

An electrochemical cell with a vent assembly that reliably prevents resealing of a vented seal member is provided. The vent assembly seals the end of a container that includes a first electrode, a second electrode, a separator located between the first and second electrodes and an electrolyte in contact with the electrodes and separator. The seal member includes a diaphragm with a hinge along one edge and a vent along another edge. The hinge and vent are located so that, relative to the longitudinal axis of the vent assembly, the hinge is closer to the vent than to the cell&#39;s terminal cover.

BACKGROUND OF THE INVENTION

[0001] This invention generally relates to ventable seals forpressurized containers and, more particularly, to ventable seals forelectrochemical cells.

[0002] Electrochemical cells, such as cylindrical alkalineelectrochemical cells, employ two electrochemically active materials andan aqueous electrolyte. The electrochemically active materials aretypically manganese dioxide and zinc. These materials are conventionallyhoused in a cylindrical elongated container that is open on one end sothat the electrochemically active materials and electrolyte can beinserted therein during the cell manufacturing process. A closureassembly that incorporates a disc shaped polymeric seal member, a rigidinner cover and an elongated metallic current collector that projectsthrough the center of the seal member, closes the open end of thecontainer. The seal member usually includes a hub, which surrounds thecollector, and a thin diaphragm integrally molded into the centralregion of the seal body. The function of the diaphragm is to rupture andrelease gas from within the cell when the internal pressure becomes toohigh. The collector provides a conductive path between the zinc and oneof the cell's terminal covers which is located on an end of the cell.

[0003] Manufacturers of electrochemical batteries constantly strive toimprove the performance of their products in a wide variety of batterypowered devices. A key objective of most battery manufacturers is toimprove the amount of time that a battery will power a device. Onestrategy to accomplish this objective is to minimize the volume occupiedby the cell's nonelectrochemically active components, such as the cell'svent assembly, thereby leaving more volume within the cell available forthe electrochemically active components. In recent years, ventassemblies have been made more volume efficient by redesigning them torequire fewer components and yet operate more reliably than previouslyknown vent assemblies. As the volume occupied by the vent assembly hasdecreased, the problem of a vented seal “resealing” against anothercomponent of the vent assembly has become more apparent. This problemcan occur in primary batteries, also known as nonrechargeable batteries,if they are charged by other batteries. While most batteries are used ina conventional manner, a small percentage of batteries are exposed to anabusive condition such as when a primary battery is inadvertently placedin a device such that the battery's positive and negative terminals arereversed. This problem could occur, for example, in some commerciallyavailable flashlights that are powered by eight AA size alkalinebatteries. Typically, the batteries are inserted into two elongatedchambers containing four batteries each. According to the instructionsthat accompany the flashlight, the batteries must be inserted “inseries,” which means that the positive terminal of one battery contactsthe negative terminal of an adjoining battery. Unfortunately, someconsumers fail to comply with the instructions that are normallyincluded with the device and accidentally insert one of the cellsbackwards. If this occurs with the second or third battery in a cavitycontaining four batteries, then the cell inserted backwards will becharged by one or more of the properly inserted cells when the light'sswitch is moved to the “on” position. Charging the battery that has beeninserted backwards generates significant quantities of heat and gaswithin the battery. In some situations, the external temperature of thebattery could exceed 65° C. The increase in temperature increases thepressure within the cell. In addition to increasing the temperature ofthe battery, the chemical reactions that take place during the chargingof the cell generate gas that substantially increases pressure withinthe cell. The simultaneous production of gas and increase in temperaturecauses the polymeric seal, which is typically made of nylon, to becomesoft and lose some of its structural rigidity. The thin ventable portionof the seal may become elongated due to both the heating of the nylonand the increase in internal pressure. Consequently, when the softenedand distorted seal ruptures in response to the pressure buildup, aninitial quantity of gas may escape from within the cell but the tear inthe ruptured seal may be resealed when the softened ruptured sealcontacts the smooth inner surface of the terminal cover and resealsagainst the terminal cover. This problem is particularly acute with lowvolume vent assemblies wherein the distance between the seal member andother components, such as the cell's cover, is very small. If theruptured seal does reseal against the cover and the cell continues togenerate gas, the cell may eventually experience a crimp release whereinthe crimped connection between the seal and container is broken and thevent assembly is forcefully ejected from the container.

[0004] As disclosed in U.S. Pat. No. 6,270,919 B1, previous attempts toprevent resealing of a ruptured seal body have included modifying aseal's inner disc portion to include ribs. The ribs are designed tomaintain the opening in a ruptured seal body thereby preventingresealing of the vent mechanism. However, while the inclusion of ribs inthe seal's diaphragm is helpful in preventing resealing in most cells,some seals with the ribs incorporated therein may be deformed whenexposed to the heat generated during charging such that the ribs cannotmaintain the opening in the seal after it has ruptured.

[0005] In a seal embodiment disclosed in U.S. Pat. No. 6,312,850 B1,vertical grooves were placed in the surface of a compression member thatforms a part of the seal assembly. The grooves are designed to preventresealing of a vented seal's diaphragm. The grooves create channels thatallow the gas to vent and thereby prevent resealing of the venteddiaphragm. While this embodiment does prevent resealing of the diaphragmagainst the surface of the hub, the grooves in the compression membercannot prevent resealing of the torn diaphragm against the interiorsurface of the cell's terminal cover.

[0006] U.S. Pat. No. 6,270,918 B1 discloses a seal assembly thatutilizes a seal member and an inner cover with openings incorporatedtherein. The seal member directly abuts the inner cover. An outer coveris secured to the seal assembly and forms a contact terminal of thecell. If the cell's pressure increases beyond an acceptable limit, theseal member ruptures thereby allowing gas to escape through the openingsin both the inner cover and contact terminal. While the described sealassembly does safely vent an electrochemical cell, the seal assemblyrelies upon the use of the inner cover. Unfortunately, the inner coveroccupies space within the cell that could be better used to store moreof the cell's electrochemically active materials.

[0007] Therefore, there exists a need for an inexpensive and simple tomanufacture polymeric vent assembly that occupies a minimum amount ofvolume within the cell and can reliably prevent resealing of a ventedseal against the electrochemical cell's terminal cover.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention provides an electrochemical cell with avent assembly that reliably prevents resealing of a vented seal member.The vent assembly occupies a minimum amount of space within the cell andis inexpensive to produce.

[0009] In one embodiment, the electrochemical cell of the presentinvention includes an open ended container that includes a firstelectrode in contact with the container, a separator in contact with thefirst electrode, a second electrode in contact with the separator andelectrolyte in contact with the electrodes and separator. A ventassembly is secured to the open end of the container. The vent assemblyhas a longitudinal axis and includes a cover, a seal member and acurrent collector. The cover is made from an electrically conductivematerial with a recessed central region. The seal member is a rupturabledisc shaped component made of a polymeric material. The seal memberincludes an upstanding wall, which defines the perimeter of the sealmember, and a thin rupturable diaphragm region that has a first edge anda second edge and is located between the seal member's upstandingperimeter wall and a centrally located hub. The hub defines an openingthrough the seal member. The seal member also includes a hinge which islocated adjacent the first edge of the diaphragm region and a vent whichis located adjacent the second edge of the diaphragm region. The hinge'selevation, relative to the longitudinal axis of the vent assembly, iscloser to the elevation of the vent than to the elevation of the cover'scentral region. The current collector is an electrically conductiveelongated component that extends through the central opening in the sealmember and contacts the cell's second electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a top view of a nonvented seal member useful in a cellof this invention;

[0011]FIG. 2 is a cross-sectional view of a nonvented seal member usefulin a cell of this invention;

[0012]FIG. 3 is a longitudinal cross-sectional view of anelectrochemical cell of this invention including the seal member shownin FIG. 1;

[0013]FIG. 4 is a top view of a vented seal member useful in a cell ofthis invention;

[0014]FIG. 5 is a cross-sectional view of a vented seal member useful ina cell of this invention;

[0015]FIG. 6 is a longitudinal cross-sectional view of anelectrochemical cell of this invention including the vented seal membershown in FIG. 5; and

[0016]FIG. 7 is an example of an electrochemical cell which is known inthe prior art and includes a vent assembly with a support member.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to the drawings and more particularly to FIG. 3,there is shown a cross-sectional view of an assembled electrochemicalcell of this invention. Beginning with the exterior of the cell, thecell components are the container 10, first electrode 12 positionedadjacent the interior surface of container 10, separator 14 contactingthe interior surface 16 of first electrode 12, second electrode 18disposed within the cavity defined by separator 14 and vent assembly 20secured to container 10. Container 10 has an open end 22, a closed end24 and a sidewall 26 therebetween. The closed end 24, sidewall 26 andvent assembly 20 define a cavity in which the cell's electrodes arehoused.

[0018] Shown in FIGS. 1 and 2 is an embodiment of seal member 28 whichhas several sections that perform a variety of functions. As shown inFIG. 3, the perimeter of seal member 28 is defined by upstanding wall 34which forms an electrically insulating barrier between container 10which serves as the current collector for first electrode 12, andterminal cover 32 which serves as the external contact for secondelectrode 18. The interior surface of perimeter wall 34, also referredto herein as the upstanding wall, forms one side of a cover receptionchannel 76 in seal member 28 into which the rollback edge 36 of cover 32is inserted. Located adjacent the interior surface of rollback edge 36is buttress wall 38 which extends above diaphragm region 40. Thefunction of buttress wall 38 is to oppose the force exerted on diaphragmregion 40 when the internal generation of gas increases pressure withinthe cell. In a preferred embodiment, diaphragm region 40 includes anelbow 42 which joins two annular sections of diaphragm 40 at an obtuseangle. By configuring the diaphragm so that it is bowed toward theinterior of the cell, the pressure exerted on the diaphragm by thebuildup of a gas within the cell results in the diaphragm beingcompressed. When the cell's internal pressure exceeds a predeterminedvalue, the diaphragm inverts and tears at vent line 44 which is a groovein the interior surface of a seal member 28. Vent line 44 is alignedwith the outer circumferential surface of compression bushing 46, sothat compression bushing 46 facilitates tearing of the diaphragm at ventline 44.

[0019] When diaphragm 40 responds to an excessive increase in internalpressure by inverting and tearing at vent line 44, diaphragm 40 forms avent flap 48, see FIGS. 5 and 6, with a distal end 50 and a proximateend 52. Vent flap 48 is specifically designed to rotate toward terminalcover 32 so that an opening 78 is created in seal member 28. Pressurizedgas within the cell escapes to the environment beyond the cell bypassing through the vented opening in the seal member and then throughopening 54 in terminal cover 32. Vent flap 48 is made to rotate aroundhinge area 56, which is located at the proximate end 52 of vent flap 48.In a preferred embodiment, hinge area 56 forms an arc that is concentricwith the central opening 30 in seal member 28.

[0020] Hinge 56 is the arc shaped line in the seal member about whichthe vent flap rotates when the seal is ruptured. The hinge coincideswith at least a portion of first edge 64 of diaphragm 40. Preferably,the hinge forms an arc or a circle that is concentric with the ventassembly's longitudinal axis. Abutting and defining another edge ofdiaphragm 40 is second edge 66. First edge 64 and second edge 66 areconcentric with one another and the vent assembly's longitudinal axis.

[0021] Preferably, second edge 66 comprises a grooved section and anongrooved section. The grooved section defines the location of vent 44.The nongrooved section forms a nonventable interface between a portionof diaphragm 40 and hub 70. The nonventable interface forms anchor 82.The function of anchor 82 is to secure vent flap 48 to both diaphragmregion 40 and hub 70, thereby ensuring that vent flap 48 does notcontact terminal cover 32 and form a 360° seal against the interiorsurface of the terminal cover when the cell's internal pressure causesseal member 28 to rupture at vent line 44. The arc formed by vent line44 should be at least 180°, more preferably approximately 220° and mostpreferably not more than 250°. The portion of diaphragm 40 that abutshub 70 and does not include vent line 44 forms an anchor 82. The arcformed by anchor 82 should be at least 30°, more preferablyapproximately 100° and most preferably less than 180°. Upon activationof the vent, opening 78 is formed by tearing the diaphragm at vent line44. Opening 78 may extend from one end of vent line 44 to the other endof vent line 44 provided opening 78 does not extend into the nongroovedsection of second edge 66. Alternatively, opening 78 may extend around aportion of vent line 44 as shown in FIG. 4. The size of opening 78 isnot critical provided it is large enough for the gas within the cell torapidly escape when the cell vents. The arc of anchor 82 and the arc ofvent line 44 form a complete circle.

[0022] Relative to the elevation of the vent assembly's longitudinalaxis 58, the elevations of hinge 56, terminal cover 32 and vent line 44in seal member 28 are critical to preventing the resealing of vent flap48 against the interior surface of cover 32. If the distal end 50 ofvent flap 48 is allowed to contact cover 32 and the cell continues togenerate gas after the diaphragm has ruptured, as would occur when acell has been inserted backwards in a multicell device and issubsequently charged by the other cells, then the flap is forcefullypressed against cover 32 thereby blocking further release of gas throughthe seal member and cover. If the internal pressure exceeds thecontainer's ability to remain crimped over the perimeter of the sealmember, then the vent assembly may be rapidly expelled from thecontainer. Some of the cell's other components, such as electrolyte andportions of the anode, may also be forcefully thrown from the container.

[0023] To prevent resealing of a vented seal member, the elevation ofthe hinge must be closer to the elevation of the vent than to theelevation of the cover's central portion. The elevations of the hinge,vent and cover are determined relative to the longitudinal axis 58 ofvent assembly 20. As shown in FIG. 3, the longitudinal axis 58 of thevent assembly is concentric with the longitudinal axis of container 10.For use in this application, the highest elevation within the ventassembly is the surface of the terminal cover's central portion. Anypoint along the vent assembly's axis that is between the terminal coverand the end of current collector 30 that contacts second electrode 18 isconsidered to be lower than the highest elevation. The elevation of anyfeature in the vent assembly is determined by connecting a straight linefrom the feature in question to the vent assembly's longitudinal axisprovided the axis and straight line meet at a right angle. Whencomparing the elevation of any two features in the vent assembly, thefeature with its straight line closest to the surface of terminal cover32 is considered to be higher in elevation than the other feature.

[0024] As shown in FIG. 3, the elevations of cover 32, hinge 56 and vent44, relative to the longitudinal axis of the vent assembly, correspondto points A, B and C, respectively. In the vent assembly of the presentinvention, the elevation of hinge 56 (point B) must be closer to theelevation of vent 44 (point C) than to the elevation of the junction ofcollector 30 and the cover's central portion (point A). Preferably, theelevation of hinge 56 is greater than the elevation of vent 44. Thephysical relationships of the cover, hinge and vent are necessary toensure that the distal end 50 of vent flap 48 does not contact andreseal against cover 32 when excessive pressure within the cell causesseal member 28 to rupture. During the rupturing of seal member 28,diaphragm 40 rapidly inverts and tears at vent line 44. The portion ofthe diaphragm located between hinge 56 and vent 44 forms the vent flap48 which is forced to rotate through the vent deflection zone, towardcover 32, by the pressurized gas as it escapes from the cell. As usedherein, the vent deflection zone is the space between diaphragm 40 andterminal cover 32. In FIG. 3, the vent deflection zone is identifiedwith parallel diagonal lines. To ensure proper operation of the ventassembly, the vent deflection zone should not be obstructed with anyobjects that would impede movement of the vent flap. For example, theexistence of a support member 62, shown in FIG. 7, would inhibit properoperation of the vent flap. Consequently, in a preferred embodiment, thevent assembly of this invention includes a seal member, a currentcollector and a single cover that also functions as one of the cell'sterminals.

[0025] Inwardly disposed from second edge 66 is hub base 68, hub 70 andcollector 30. Hub base 68 is thicker than diaphragm 40. Hub 70 abuts hubbase 68 at a right angle. Hub 70 defines central opening 30 in sealmember 28.

[0026] Seal member 28 is typically made as a single component byinjection molding a polymeric material into a cavity and then allowingthe part to solidify. Materials from which the seal member may be madeinclude: nylon 6, 6; polypropylene; and impact modified polystyrene asdescribed in U.S. Pat. No. 6,306,537.

[0027] An optional component of the vent assembly shown in FIGS. 3 and 6is compression brushing 46. The exterior circumferential surface 72 ofthe compression bushing is made to align with vent 44. In ventassemblies that do not include a compression bushing, the vent is madeto align with the outside diameter of hub 70.

[0028] Terminal cover 32 is a generally cup-shaped component made ofnickel-plated steel. The perimeter 36 of cover 32 is formed by rollingback the edge of the cover upon itself to create a double thickness ofmaterial that is approximately perpendicular to the central portion 74of cover 32. Between the covered central portion and its perimeter, thecover is contoured through a series of bends. The cover's central region74 is recessed relative to the cover's rolled back edge 36. When cover32 is inserted into the seal member's cover reception cavity 76 and thevent assembly is crimped into the open end of container 10, the coveracts like a spring that compresses the seal member's upstanding wall 34against the interior surface of container 10. By contouring the cover topossess a spring like characteristic, the vent assembly does not need toinclude a separate component to force the seal member's perimeter wallagainst the container.

[0029] First electrode 12 is a mixture of manganese dioxide, graphiteand an aqueous solution containing potassium hydroxide. The electrode isformed by disposing a quantity of the mixture into the open endedcontainer and then using a ram to mold the mixture into a solid tubularshape that defines a cavity which is concentric with the sidewall of thecontainer. Alternatively, the cathode may be formed by preforming aplurality of rings from the mixture comprising manganese dioxide andthen inserting the rings into the container to form the tubularly shapedfirst electrode.

[0030] Second electrode 18 is a homogenous mixture of an aqueousalkaline electrolyte, zinc powder, and a gelling agent such ascrosslinked polyacrylic acid. The aqueous alkaline electrolyte comprisesan alkaline metal hydroxide such as potassium hydroxide, sodiumhydroxide, or mixtures thereof. Potassium hydroxide is preferred. Thegelling agent suitable for use in a cell of this invention can be acrosslinked polyacrylic acid, such as Carbopol 940®, which is availablefrom Noveon, Inc., Cleveland, Ohio, USA. Carboxymethyylcellulose,polyacrylamide and sodium polyacrylate are examples of other gellingagents that are suitable for use in an alkaline electrolyte solution.The zinc powder may be pure zinc or an alloy comprising an appropriateamount of one or more of the metals selected from the group consistingof indium, lead, bismuth, lithium, calcium and aluminum. A suitableanode mixture contains 67 weight percent zinc powder, 0.50 weightpercent gelling agent and 32.5 weight percent alkaline electrolytehaving 40 weight percent potassium hydroxide. The quantity of zinc canrange from 63 percent by weight to 70 percent by weight of the anode.Other components such as gassing inhibitors, organic or inorganicanticorrosive agents, binders or surfactants may be optionally added tothe ingredients listed above. Examples of gassing inhibitors oranticorrosive agents can include indium salts (such as indiumhydroxide), perfluoroalkyl ammonium salts, alkali metal sulfides, etc.Examples of surfactants can include polyethylene oxide, polyethylenealkylethers, perfluoroalkyl compounds, and the like.

[0031] The second electrode may be manufactured by combining theingredients described above into a ribbon blender or drum mixer and thenworking the mixture into a wet slurry.

[0032] Electrolyte suitable for use in a cell of this invention is athirty-seven percent by weight aqueous solution of potassium hydroxide.The electrolyte may be incorporated into the cell by disposing aquantity of the fluid electrolyte into the cavity defined by the firstelectrode. The electrolyte may also be introduced into the cell byallowing the gelling medium to absorb an aqueous solution of potassiumhydroxide during the process used to manufacture the second electrode.The method used to incorporate electrolyte into the cell is not criticalprovided the electrolyte is in contact with the first electrode 12,second electrode 18 and separator 14.

[0033] During the cell assembly process, a quantity of an aqueouspotassium hydroxide solution is incorporated into the cell. The solutionserves as the cell's electrolyte. The electrolyte may be incorporatedinto the cell by allowing the gelling agent in the second electrode toabsorb the electrolyte during the electrode manufacturing process whichoccurs prior to inserting the second electrode into the coiledseparator. In addition, a quantity of electrolyte may be dispensed intothe partially completed cell any time after the first electrode has beeninserted into the container and prior to closing the cell.

[0034] Separator 14 may be made from nonwoven fibers. One of theseparator's functions is to provide a barrier at the interface of thefirst and second electrodes. The barrier must be electrically insulatingand ionically permeable.

[0035] The above description is considered that of the preferredembodiments only. Modifications of the invention will occur to thoseskilled in the art and to those who make or use the invention.Therefore, it is understood that the embodiments shown in the drawingsand described above are merely for illustrative purposes and are notintended to limit the scope of the invention, which is defined by thefollowing claims as interpreted according to the principles of patentlaw, including the Doctrine of Equivalents.

What is claimed is:
 1. An electrochemical cell, comprising: (a) an open ended container, said container comprises a first electrode in contact with said container, a separator in contact with said first electrode, a second electrode in contact with said separator and electrolyte in contact with said electrodes and said separator; and (b) a vent assembly, having a longitudinal axis, is concentrically aligned with and secured to the open end of said container, said vent assembly comprises: i) an electrically conductive cup shaped cover having a recessed central region; ii) a disc shaped rupturable seal member made of a polymeric material and contacting said cover, said seal member comprises an upstanding wall defining the perimeter of the seal member, and a thin rupturable diaphragm region having a first edge, a second edge and located between the seal member's upstanding perimeter wall and a centrally located hub which defines an opening through said seal member, said seal member comprises a hinge located adjacent a first edge of the thin diaphragm region and a vent located adjacent a second edge of the thin diaphragm region, said hinge's elevation, relative to the longitudinal axis of the vent assembly, is closer to the elevation of the vent than to the elevation of the cover's central region; iii) an electrically conductive current collector extends through the opening in said seal member and contacts said second electrode.
 2. The electrochemical cell of claim 1 wherein said second edge of said thin diaphragm region comprises a grooved section and a nongrooved section, said grooved section defining the location of said vent and said nongrooved section defining an anchor which secures said diaphragm region to said hub.
 3. The electrochemical cell of claim 2, wherein said groove is arc shaped and concentrically aligned with the central opening in said seal member.
 4. The electrochemical cell of claim 2, wherein said nongrooved section forms at least a 30° arc.
 5. The electrochemical cell of claim 2, wherein said nongrooved section forms approximately a 100° arc.
 6. The electrochemical cell of claim 2, wherein said nongrooved section forms less than a 180° arc.
 7. The electrochemical cell of claim 2 wherein said grooved section forms an arc greater than 180°.
 8. The electrochemical cell of claim 2 wherein said grooved section forms an arc greater than 220°.
 9. The electrochemical cell of claim 2 wherein said grooved section forms an arc greater than 250°.
 10. The electrochemical cell of claim 1 wherein said hinge's elevation is between the elevation of the vent and the elevation of the cover's central region.
 11. The electrochemical cell of claim 1 wherein said hinge is arc shaped and concentrically aligned with the central opening in said seal member.
 12. The electrochemical cell of claim 1 wherein said seal member comprises a buttress wall abutting said diaphragm region and said cover when said cell is pressurized, said buttress wall positioned to resist the compressive force exerted on said diaphragm by an increase in the cell's internal pressure.
 13. The electrochemical cell of claim 1 wherein said polymeric material is selected from the group consisting of nylon, polypropylene and impact modified polystyrenes. 